While the immune system is adept at recognizing and neutralizing the effects of various pathogens (e.g., bacteria, viruses, fungi, protozoa, and metazoa) and mutated self-cells (e.g., pre-cancer and cancer cells), failure of the immune system to perform its functions often results in disease (e.g., infection and cancer).
Infection of humans with various pathogens continues to present a serious problem with significant clinical and economic consequences. Treatment of many human infections is either problematic (treatment-resistant strains, toxicity of the treatment, drugs not reaching well the infection site, etc.) or practically nonexistent (e.g., viral infections).
To date, cancer remains the single most common cause of morbidity and mortality of humans. Using animal models, the art has long recognized that protective immunity against cancer could be achieved by first exposing the animal to tumor cells in a non lethal manner. These early studies showed that tumors could be categorized as immunogenic or nonimmunogenic based on their ability to induce immunity to a subsequent challenge. Furthermore, tumor antigenicity appeared to be clonal in nature, such that immunization with a certain tumor elicited an immune response capable of rejecting only that same tumor. In addition, immunological memory and tumor rejection were determined to be mediated by T lymphocytes.
Much effort has focused on the identification of tumor-associated antigens. To date, the majority of tumor-associated antigens represent major histocompatibility (MHC) class I-restricted T cell epitopes. These epitopes are generally derived from proteins which have undergone mutation during tumorigenesis, which are normally expressed at a stage in embryogenesis preceding development of the immune system, or are present in overabundance in cancerous tissue compared to normal tissue (Urban and Schreiber (1992) Ann. Rev. Immunol. 10:617-644).
Although identification of these tumor-associated antigens was valuable to the study of tumor immunogenicity, identification of these antigens did not provide a suitable approach to the prevention or treatment of a wide spectrum of cancers since these antigens were rarely shared between tumors of distinct origin. Furthermore, for tumor clearance and long-term protection against tumor growth, expression of tumor-associated antigens further requires recruitment of T cells to its location, presentation of the antigenic peptides to tumor-specific T cells, and the provision of co-stimulatory signals.
One of the approaches to enhancing tumor antigenicity has focused on enhancing T cell recruitment and activation by direct genetic modification of tumor cells to express various cytokines (such as interleukin-2 (IL-2) or interleukin-12 (IL-12)) which function in the recruitment and activation of T cells, and which are also enhanced if high levels of myeloid chemokines are present at the site of the tumor. Chemokines recruit "professional" antigen presenting cells such as macrophages and dendritic cells, which then provide the co-stimulatory signals required for T cell activation. An alternative approach to enhancing tumor antigenicity has employed direct genetic modification of tumor cells to express costimulatory molecules (such as B7-1 or B7-2) and hence to enhance primary anti-tumor T cell responses.
While expression of cytokines and costimulatory molecules has been effective in enhancing primary-anti-tumor T cell responses, such expression does not restore immunogenicity of poorly immunogenic tumors. This result is explained, in part, by the defective presentation by many poorly immunogenic murine and human cancers of endogenous antigens as a result of a diminished synthesis of major histocompatibility complex class I, proteasome components or peptide transporters associated with antigen processing (Restifo et al. (1991) J. Immunol. 147:1453-1459; Restifo et al. (1992) J. Exp. Med. 175:1423-1431; Restifo et al. (1993) J. Exp. Med. 177:265-272; Kuroda et al. (1995) Immunology 84:153-158; Kaklamanis et al. (1995) Cancer Res. 55:5191-5194). Similarly, while the immunogenicity of poorly immunogenic tumors can in some instances be restored by IFN-.gamma. treatment (Restifo et al. (1992) J. Exp. Med. 175:1423-1431), The therapeutic effect of IFN-.gamma. is unpredictable since the susceptibility of cells within a malignant clone as well as that of various tumors is very variable. Indeed, many cells simply will not upregulate the MHC class I molecules when exposed to IFN-.gamma..
Another approach to enhancing tumor antigenicity has relied on transfection of tumor cells with an MHC class I gene under the control of a heterologous promoter (Tanaka et al. (1988) Mol. Cell. Biol. 8:1857-1864). However, this method does not facilitate peptide-loading and therefore expression of functional MHC-peptide complexes is expected to be low. In addition, this method is not very effective in cells which already express a sufficient amount of MHC.
Yet another approach to enhancing the antigenicity of tumors is by fusion of tumor cells with activated B cells (Guo et al. (1994) Science 263:518-520). However, restoration of immunogenicity is not uniform in tumors from different tissues or different types of tumors from the same tissue. This lack of uniformity is the result, in part, of the random retention by tumor cells of desirable chromosomes (i.e., chromosomes which contain the tumor-antigen coding genes and genes coding for the co-stimulatory or immuno-potentiating principle).
Thus, there remains a need for methods for the prevention and treatment of a wide spectrum of infectious pathogens and of cancers, and in particular, poorly immunogenic pathogens and cancers.